call: Add unknown state

This is the default fallback state and is useful f.e. in the oFono plugin
if the GDBOVoiceCall doesn't provide a state in its' properties.

src/calls-call.h

@@ -37,7 +37,8 @@ G_DECLARE_DERIVABLE_TYPE (CallsCall, calls_call, CALLS, CALL, GObject)
 
 typedef enum
 {
-  CALLS_CALL_STATE_ACTIVE = 1,
+  CALLS_CALL_STATE_UNKNOWN = 0,
+  CALLS_CALL_STATE_ACTIVE,
   CALLS_CALL_STATE_HELD,
   CALLS_CALL_STATE_DIALING,
   CALLS_CALL_STATE_ALERTING,